Heuristics has a number of patterns for common headings, but it only handles chapter headings like you're describing if the headings are all in capitals - e.g. ONE, TWO, THREE, etc. If they're a mix of uppercase/lowercase characters then it's basically indistinguishable from lines of poetry and other random unpunctuated text that can appear in a book. Right now heuristics should still insert page breaks at each of those points, which should make it easier to do final edits in Sigil.

As for what code to put there, that depends on how chapters are marked in the book (as in what XHTML code is used to represent a chapter heading). There's a tutorial available in the FAQ, but without actual code samples from the book, we won't be able to help you with that.